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GTC optical observations of SGR 0418+5729

ATel #2244; R. P. Mignani (MSSL-UCL), N. Rea (ICE-CSIC, IEEC), V. Testa, G. L. Israel, S. Covino, L. Stella, S. Mereghetti, M. Burgay, A. Possenti (INAF), S. Chaty (CEA-Saclay), P. Jonker (SRON), G. Lo Curto (ESO), R. Perna (U. Boulder), R. Turolla (U. Padova), S. Zane (MSSL-UCL)
on 16 Oct 2009; 18:44 UT
Credential Certification: Nanda Rea (N.Rea@sron.nl)

Subjects: Optical, Neutron Star, Soft Gamma-ray Repeater, Pulsar

We observed the field containing the new SGR 0418+5729 (ATel #2076, #2077) in the i'-band on 2009 September 15 with the OSIRIS camera (0.25"/pixel) mounted at the 10.4m Gran Telescopio Canarias (GTC) in the La Palma Observatory, as a part of our magnetar ToO observing program.

We took 25 dithered exposures of ~100s each , for a total integration time of ~2500s, under dark time and with average seeing conditions of ~1.3" (airmass 1.3-1.5). Photometry calibration was performed using observation of a Landolt standard star field. The image astrometry was computed against 2MASS stars, resulting in an accuracy of ~0.1" on both RA and Dec. We did not detect a source within the 95% Chandra confidence error-circle of SGR 0418+5729 (Atel #2159), down to a 3 sigma limiting magnitude of i' = 25.1 .

De-reddening the measured upper limits using the value of the hydrogen column density derived from the X-ray spectral fits (NH = 1.6e21 cm^-2), and using the unabsorbed 1-10 keV X-ray flux at an epoch close to our GTC observations, we infer an X-ray versus optical flux ratio of F_X/F_opt > 20000 .

The GTC i'-band finding chart can be found at: http://staff.science.uva.nl/~nrea/gtc_sgr0418_i.tif

The radius of the error circle (0.5") accounts for the 0.35" uncertainty on the Chandra position of SGR 0418+5729 (Atel#2159) and for the accuracy of our astrometry calibration.

We wish to thank Rene Rutten, Riccardo Scarpa and the whole GTC staff for their help and prompt reaction.

GTC i' finding chart